In general, a superconductor is used while being cooled in a forced method or an indirect method by immersing it in a refrigerant such as liquid helium or combining it with a refrigerator or the like to maintain its superconductivity. Specifically, a composite superconductor in which aluminum and a superconductor composed of an alloy superconductive material such as NbTi are combined so as to take advantage of high specific heat, high thermal conductivity, adjustable electric conductivity, small specific gravity, low radioactivity and the like of aluminum is put into practical use (see, for example, Japanese Patent Application Laid-Open (JP-A) No. 2000-164053).
Further, JP-A 2001-267120 introduces application of joining by friction stir welding (FSW) in order to produce a superconducting coil by joining aluminum covering conductors combined with the alloy superconductive material.
Further, JP-A 2007-214121 discloses a high performance composite superconductor comprising, as a superconductor, a compound superconductive material or the like having excellent superconducting characteristics such as critical current density, critical magnetic field, and critical temperature, wherein the thermal, mechanical, and electrical contact conditions between the superconductor and the metal member such as aluminum are controlled.
In general, a compound superconductor is produced by subjecting a raw material to a generation treatment such as a heat treatment, and is vulnerable to mechanical strain. Therefore, when using a production method such as coating extrusion or composite wire drawing which has been applied to a superconductor of alloy superconductive material, the critical current characteristic is partially decreased due to application of plastic working, which is problematic. In order to solve this problem, friction stir welding (FSW) is applied as a joining method for combining a compound superconductor and aluminum. Although, conventionally, there is a method in which an Nb3Sn superconductor is arranged at a hollow portion formed by a metal member obtained by combining two copper members and a joint portion of the copper members is soldered (for example, see Cryogenic engineering 39, vol. 9, 2004, pages 383 to 390, ANDO Toshinari), it was difficult to change the metal member to aluminum. However, by applying FSW, it becomes possible to use, as a metal member, an aluminum member in place of the copper member. Since aluminum has a large thermal conductivity and a high specific heat, and the surface thereof is easily to be oxidized, it is necessary to rapidly apply a large amount of heat in a state in which the oxide film on the surface thereof is removed when soldering is carried out. However, by applying FSW, application of a large amount of heat can be avoided.
JP-A H09-069318 discloses a method in which an NbTi superconductor and a stabilizer are accommodated in a hollow portion formed by combining two strengthening members, and the strength members are welded with each other by arc welding or beam welding.
However, since, in a conventional method such as soldering, brazing, arc welding (TIG welding or MIG welding) or beam welding, adjustment of the quantity of heat to be applied to a metal member during welding is difficult, there has been a risk of decrease in dimensional accuracy of the joint portion, or deformation or alteration in quality of a superconductor due to heat during welding. Since, in FSW, welding is performed not by fusion joining but by solid phase joining, the above-mentioned problems in conventional art are expected to be overcome.